Attempts have been made in pursuit of better gripping performance of such grip structures as those mentioned above. One example of them is a research and development project involving contrivances regarding the hardness, surface state and shape of the grip portion.
However, since a grip portion of an elastic member is usually fixed to the shaft cylinder, there is a limit to the satisfaction of the gripping person on account of the size of the gripping hand and his or her personal preference.
In order to solve this problem, an inventive method was suggested by which the grip is formed to match the shape of the user's fingers (see Reference 1: Japanese Patent Application Laid-Open 2001-001681, (Claim 1 and FIG. 1). However, as this would require an impression material and a mold, it would be expensive and, moreover, not only the molding work is troublesome but also the writing tool, though good for its user, would result in an exclusive use for only him or her (the user).
Then, there was made an attempt fir partially expanding or contracting the grip portion by turning the shaft cylinder (see Reference 2: Japanese Patent Application Laid-Open 2000-313193 (Claim 1, FIG. 1 and FIG. 2)).
However, according to Reference 2 cited above, though the grip portion can be expanded or contracted in the radial direction, it still left room for further development. Thus, according to Reference 2, as the expansion or contraction takes place uniformly in the circumferential direction, specific fingers are caused to feel unpleasant. Thus, a writing tool is usually gripped with three fingers including the thumb, index finger and middle finger. If, however, the grip portion is expanded (contracted) and adjusted mainly on the basis of the thumb, the index finger and middle finger will feel unpleasant.
In addition to the above, there were a wide variety of requirements regarding the relative hardness (hardness or softness) of the grip portion and, as a user mainly concerned about the relative hardness tends to purchase a product of a relative hardness meeting his or her preference, this again would result in a writing tool for only an exclusive use by that particular person (user).